Photochemistry on rough metal surfaces

Abstract

In this paper the general question of laser-induced photochemistry on metal surfaces is addressed. Specifically, we have studied resonant photodecomposition of a variety of aromatic molecules on roughened silver surfaces in ultrahigh vacuum. A continuous ion laser source at a number of different wavelengths in the region 350-410 nm was used to produce graphitic carbon on the surface which was monitored by Raman spectroscopy at the 1580-cm"1 band of surface carbon. Laser power-dependence studies of fragmentation rate for several molecules at 406.7 nm indicate that the initial absorption step is a two-photon process. Energetic considerations imply that photochemistry for other molecules studied is also due to multiphoton absorption, except for benzaldehyde fragmentation with 350.7-nm excitation where the photodecomposition rate is linear in intensity. In this case photochemistry apparently occurs directly from the first excited singlet state following one-photon absorption. Distance-dependence studies of photofragmentation rates by use of an inert spacer layer to separate the molecule undergoing photochemistry from the surface indicate that energy transfer to the metal surface is important in determining the reaction rate. A maximum decomposition rate is observed for pyridine 15-20 Á from the surface, resulting from the different distance dependences of enhanced molecular absorption at rough silver surfaces and damping by energy transfer to the surfaces. These competing processes produce a maximum photochemical rate where surface-enhanced absorption is still present but the energy damping rate has decreased substantially. Intermediate species in the decomposition of pyridine and pyrazine to carbon have been observed as CN peaks in the surface Raman spectrum during photolysis of these molecules. The CN stretch frequency of these fragments is shifted from that observed for cyanide ions adsorbed on silver to a frequency typical of a nitrile such as C3H3N. The fragmentation mechanism for these and other surfaces adsorbed molecules studied remains to be established. Evidence indicates that radical formation is a likely pathway for some molecules (e.g., benzaldehyde) while other molecules may decompose through surface ionic species.

Photolysis of ClNO adsorbed on MgO(100)

“…Additionally, interference between the incident, transmitted, and reflected radiation was not taken into account (12). No pronounced surface-enhanced photodissociation was seen, as has been reported for rough metal surfaces (36,37) and for OCS on unannealed LiF(OO1) (1 2a,b). We also note that the QMS samples a small solid angle (-5.5"), while for accurate cross section measurements all particles must be detected, i.e., angular distributions may vary with coverage, as noted by Polanyi and co-workers (9).…”

Morphological Characteristics of Au Films Deposited on Ti: A Combined SEM-AFM Study

“…In this regard, in particular, the nanoscale control of the Au film structure is crucial towards the determination of its properties in view of applications in functional nano-devices. For example, the roughness of deposited metal films is one of the most critical determining final electrical, optical and other properties of the systems [16][17][18][19][20][21][22][23][24][25]. As examples: (a) one of the key issue in the modern integrated circuit technology is the dramatic increase in metallic interconnect resistivity with decreasing cross section thickness.…”

“…As a consequence, the roughness determines the absorbing and scattering properties of the metal film. By controlling these properties, thin metal films are exploited as light absorbing or scattering elements in plasmonic solar cells or other optical devices [20][21][22][23]; (d) laser-induced photochemistry on metal surfaces is widely studied as an effective means to catalyze surface reactions for specific adsorbates [24]. However, the surface roughness of the metal film was proved to greatly impacts on the rate of the reactions by controlling the light absorption in the metal film; (e) surface roughness dictates, also, the surfaces wetting properties and its nanoscale control is essential for designing and controlling wetting processes in general.…”

Five Orders-of-Magnitude Enhancement of Two-Photon Absorption for Dyes on Silver Nanoparticle Fractal Clusters

“…Photodecomposition of pyridine and other aromatic molecules on roughened Ag(110) surfaces has also been attributed to an enhanced two-photon absorption process. 12 Much larger enhancements of nonlinear optical effects and TPA, in particular, are expected to occur near fractal clusters of metal nanoparticles. 4 Because an applied electromagnetic field can excite collective oscillations of free electrons (plasmons) of metal nanostructures, large local fields can be generated.…”